Combi boilers heat water on demand and don't need a cylinder. System boilers heat a stored cylinder, so you have hot water ready when you need it. Here's how they compare and which one suits your house.
Combi boilers
A combi (combination) boiler heats water directly from the mains when you turn on a tap. No cylinder, no cold water tank, just the boiler.
Pros
- Cheaper to install (no cylinder needed)
- Takes up less space (just the boiler unit)
- Hot water on demand (never runs out)
- Good mains pressure (if your incoming mains is decent)
Cons
- Limited flow rate (struggles with multiple taps running at once)
- Performance depends on mains pressure and flow rate
- No backup if the boiler breaks (you lose heating and hot water)
Best for
Small to medium houses (1-3 bedrooms, one bathroom) with good mains pressure. If you rarely run multiple taps at once, a combi is simpler and cheaper.
System boilers
A system boiler heats water and stores it in a hot water cylinder (usually in an airing cupboard). When you turn on a tap, hot water comes from the cylinder, not directly from the boiler.
Pros
- Can supply multiple taps at once without losing pressure
- Works with power showers and large baths
- Stored hot water means backup if demand spikes
- Compatible with solar thermal panels (if you add them later)
Cons
- More expensive to install (cylinder costs £500 to £1,200)
- Takes up more space (cylinder in airing cupboard or loft)
- Hot water can run out if you use more than the cylinder holds
- Cylinder loses heat over time (standing heat loss)
Best for
Larger houses (4+ bedrooms, multiple bathrooms) or homes with high hot water demand. If you often run showers, baths, and taps at the same time, a system boiler performs better.
Cost comparison
| Type | Boiler cost | Cylinder cost | Total install |
|---|---|---|---|
| Combi | £800-£1,800 | £0 | £1,800-£3,500 |
| System | £900-£1,700 | £500-£1,200 | £2,200-£4,000 |
Hot water performance
Combi
Flow rate depends on boiler size and mains pressure. A typical 28kW combi delivers around 11-13 litres per minute at 35°C rise (cold mains to 40°C hot water). That's enough for one shower or one bath tap, but not both at once.
System
Flow rate depends on cylinder size and pump. A 200L cylinder can supply multiple outlets simultaneously at full pressure until the stored water runs out. Recovery time (time to reheat the cylinder) is 20-40 minutes.
Space requirements
Combi
Just the boiler (about the size of a kitchen wall cupboard). Usually mounted on a kitchen or utility room wall.
System
Boiler plus cylinder. The cylinder (typically 150-250L) goes in an airing cupboard, utility room, or loft. Needs about 0.5-0.7m² of floor space or cupboard space.
Which should you choose?
Choose a combi if:
- You have 1-3 bedrooms and one bathroom
- You don't often run multiple taps at once
- You don't have space for a cylinder
- You want the cheapest install cost
Choose a system boiler if:
- You have 4+ bedrooms or multiple bathrooms
- You regularly run showers, baths, and taps simultaneously
- You have poor mains pressure (below 2 bar)
- You might add solar thermal panels later
- You want backup hot water capacity
Converting between types
You can convert from system to combi (saves space, removes the cylinder) or combi to system (adds hot water capacity). Conversion costs more than a like-for-like replacement because of pipework changes.
Expect to add £500 to £1,500 to the install cost for a conversion vs a straight swap.
Sources
- Heating and Hotwater Industry Council (2021), "Domestic heating compliance guide", hhic.org.uk